Fiber-reinforced composite structures provide several advantages over metallic structures. For example, composite structures can be configured to provide high specific stiffness and high specific strength relative to metallic structures. Furthermore, composite structures can be tailored to provide a relatively high degree of strength and stiffness along a primary load path. The ability to tailor the strength and stiffness of composites may result in lightweight structures. In addition, composite materials may have improved fatigue resistance relative to metallic materials and may be more resistant to corrosion.
Composite structures may be formed as a laminate of relatively thin layers or plies that are laminated together. Each ply in the composite laminate may include fibers that serve as the primary load-carrying constituent. Composite plies may be formed of unidirectional tape wherein the fibers in each ply are oriented parallel to one another and are held in position by a matrix constituent such as an epoxy resin. The matrix constituent may also redistribute or transfer loads between adjacent fibers. A composite laminate may be configured such that the fibers are oriented to provide the desired strength and stiffness characteristics of the composite structure.
Composite structures may be constructed as traditional laminates, non-traditional laminates, or steered fiber laminates. Traditional composite laminates are composed of plies with constant fiber angles oriented at 0°, ±45°, and 90° relative to the primary load direction of the composite laminate. Non-traditional laminates contain one or more plies with fibers oriented at constant angles other than the traditional 0°, ±45°, and 90° angles. Steered fiber laminates contain one or more plies with fiber angles that continually vary within the plane of each ply. The ability to orient fibers at non-traditional angles and/or vary the fiber angles within the plane of the plies allows for significant improvements in the structural efficiency of a composite laminate. For example, a non-traditional laminate or a steered fiber laminate may be optimized with fiber angles that provide improved strength and/or stiffness characteristics relative to a traditional laminate of the same thickness.
The process of designing a composite laminate may include optimizing the ply layup by iteratively adjusting lamination parameters or adjusting the individual fiber angles and laminate thickness until the process converges on a ply stacking sequence that meets the strength, stiffness, weight, and manufacturing requirements of the composite laminate. The optimization process may require checks on the strength margins of safety of each composite laminate configuration during the optimization process. For certain structures, loading conditions may dictate that a composite laminate has a relatively large quantity of plies. For example, a wing panel of an aircraft may require up to one hundred or more composite plies, each of which requires the determination of the fiber angle. As may be appreciated, the process for optimizing the layup and performing numerous strength checks on a relatively thick composite laminate is preferably performed in a computationally inexpensive manner.
Existing strength check methods have certain drawbacks that detract from their overall utility. For example, one strength check method may rely on a laminate-based allowables database generated from coupon testing. Material allowables are an established limit on material capability in the design of a composite structure, and may be used as a strength margin check. Unfortunately, the use of allowables may result in an overly-conservative and unnecessarily heavy design in certain types of laminates. In addition, a significant amount of coupon testing would be required to characterize the range of possible layups for non-traditional and steered fiber laminates due to the relatively large design space provided by such laminates. Such a coupon testing program may be prohibitively expensive and time-consuming. Additionally, any new fiber-matrix material system would require the determination of material allowables for all forms of laminates.
As can be seen, there exists a need in the art for a system and method for predicting the strength characteristics of composite laminates which is computationally inexpensive and which can be efficiently applied to new fiber-matrix material systems.